Alpha-Synuclein Conditional Knock-in Mice as Novel Models of Parkinson's Disease Abstract Mutations in the alpha-synuclein gene unequivocally cause rare autosomal dominant forms of familial Parkinson's disease (PD). Alpha-synuclein is considered to play a central role in the pathogenesis of Parkinson's disease based upon the association of missense mutations or triplications of the alpha- synuclein gene with familial Parkinson's disease, as well as the identification of fibrillar forms of alpha-synuclein as a major structural component of Lewy bodies in Parkinson's disease and other alpha-synucleinopathies. Familial mutations in alpha-synuclein cause Parkinson's disease through a toxic gain-of-function mechanism. Evidence for a gain-of-function mechanism has been provided through the generation of transgenic mouse models of Parkinson's disease that express pathogenic variants of human alpha-synuclein in a neuronal-specific manner. These transgenic models exhibit combinations of motoric dysfunction, selective neuronal degeneration, and the formation of intracellular inclusions resembling Lewy bodies. However, dopaminergic neurons in the substantia nigra are surprisingly yet consistently spared. Importantly, current alpha-synuclein transgenic mice do not provide a robust model of Parkinson's disease due to the conspicuous absence of nigrostriatal dopaminergic degeneration but may instead represent useful alpha-synucleinopathy models. The lack of a robust PD-like phenotype in alpha-synuclein mice may relate to the choice of promoter employed for transgene expression, since many of these tend to drive robust transgene expression within the brainstem and spinal cord of mice leading to motor neuron-related phenotypes rather than nigrostriatal pathway-related phenotypes. Thus, robust transgenic expression of alpha-synuclein pathogenic variants specifically within nigrostriatal dopaminergic neurons would overcome this problem and would selectively target the principal neurons affected in Parkinson's disease. To achieve expression of alpha-synuclein pathogenic variants within nigrostriatal dopaminergic neurons, a conditional Cre-loxP-based knock-in strategy will be employed to generate mice that express alpha-synuclein variants (wild-type and E46K mutant) specifically within tyrosine hydroxylase (TH)-positive catecholaminergic neurons. These mice will be evaluated as a novel, robust model of Parkinson's disease. Mutations in a gene called alpha-synuclein are a rare cause of Parkinson's disease in some families. Understanding how these genetic mutations cause disease may allow the development of novel therapies aimed at halting, slowing or reversing disease progression. Our studies aim to model the pathogenic effects of mutant forms of alpha-synuclein in mice in order to accurately recreate the typical features of Parkinson's disease so that we can better understand this disease, and so that we may be able to ultimately use such mice to test novel therapies. [unreadable] [unreadable] [unreadable]